x86/asm/purgatory: Start using annotations

Purgatory used no annotations at all. So include linux/linkage.h and
annotate everything:

* code by SYM_CODE_*
* data by SYM_DATA_*

 [ bp: Fixup comment in gdt: ]

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Alexios Zavras <alexios.zavras@intel.com>
Cc: Allison Randal <allison@lohutok.net>
Cc: Enrico Weigelt <info@metux.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: linux-arch@vger.kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20191011115108.12392-16-jslaby@suse.cz

arch/x86/purgatory/stack.S

@@ -5,13 +5,14 @@
  * Copyright (C) 2014 Red Hat Inc.
  */
 
+#include <linux/linkage.h>
+
 	/* A stack for the loaded kernel.
 	 * Separate and in the data section so it can be prepopulated.
 	 */
 	.data
 	.balign 4096
-	.globl stack, stack_end
 
-stack:
+SYM_DATA_START(stack)
 	.skip 4096
-stack_end:
+SYM_DATA_END_LABEL(stack, SYM_L_GLOBAL, stack_end)